Present methods of preventing successful counterfeiting of, say, banknotes include the use of intricate designs, watermarks and inlaid metal strips, the intention being that the application of these devices to banknotes paper is sufficiently difficult to make it likely that forged notes will be readily recognizable by their poor quality. However, the effectiveness of such preventive measures is continuously being eroded as the techniques and apparatus available to the forger become more advanced and easier to operate, thus making it potentially easier to simulate the present form of banknotes.
The requirements therefore of any new authenticating security device are primarily twofold. Firstly the methods necessary for manufacturing the device or applying it to the sheet material employed should entail the use of resources which, by reason of their nature, complexity, cost or other factors, would not normally be available to a forger. Secondly, the authentic product should be readily recognizable to the eye without having to employ special apparatus.
Optical multilayer devices are presently commercially available, comprising a substrate bearing a number of stacked thin film layers of various materials. By careful selection of the thickness and composition of the layers the optical characteristics in particular of spectral reflectance and transmittance of the device can be controlled. For instance it is well known to produce accurate colour filters by depositing such thin film layers on glass substrates. Other devices can be specially made in which the substrate is a thin transparent sheet of plastics material, and which when illuminated by white light exhibit a strong reflection in a designated part of the spectrum, dependent upon the physical characteristics of the films deposited on the substrate to form the stack. Moreover the spectral reflectance of such a device can vary with the angle at which it is viewed, so that the part of the spectrum which is strongly reflected changes as the device is tilted in relation to the direction of the illuminating light. When such a device is viewed by transmitted light, a complementary colour to the main reflected colour is observed with a similar colour change on tilting. Such a device is difficult to manufacture and can be readily recognized under ordinary ambient viewing conditions. It is proposed to incorporate a device of this type for the purpose of authenticating an item of sheet material such as a banknote as mentioned earlier.